60' wood bowstring truss broken bottom chord

[efsinc]

My client has 60' span bowstring trusses in need of various repairs.

I have seen engineers utilize a rod and turnbuckle system to provide tension at the bottom chord line. The system seems sound to me.

How do I instruct the installer to achieve the required tension in the rod system? How do I obtain certification that the required tension is in fact in the rod system?

i sure appreciate any guidance or cautions you can provide to me.

 

[MiketheEngineer]

This can be done and there are other methods to make repairs. Obviously you need to do a complete analysis of the truss and find out what the maximum forces might be.

This will guide you designing the rod attachments - not usually easily done. Then you need to releive the load on the truss and install rod assembly and add some pre-tension to take out any "give" OR pretension based on load it is now seeing.

You might need to get some hands on help with this one. There are a lot of variables and things can go badly.

 

[efsinc]

Mike, understood. Is there an attachment that you are referencing? Thank you.

 

[MiketheEngineer]

No - just experience trying to fix them..

 

[JAE]

I once repaired a wood bowstring truss using twin steel tubes on each side - attached to the existing wood bottom chord with wing plates welded to new side gusset plates on the wood - through bolted and spaced out close to the existing vertical members and their gusset plate locations.

We didn't tension the tubes as we weren't looking to re-establish the trusses to a pre-deflected shape but was just looking for safety and, with the arched top chord, knew that we were OK with drainage.

I would agree to pre-shore the trusses. You could even attempt to jack up the truss to original geometry prior to installing the tension rods, or tubes, or whatever you use.

I'd be wary of using turnbuckles to tension the trusses while they are under load. Vertical shoring towers and vertical jacking seems safer and more "under control".

 

[oldrunner]

One of your problems, if you so choose to add some sort of ties each side of the existing bottom flange, will be connecting to the ends of the existing truss to resist and be equal to the compressive from the top chords. The truss should now be shored before you begin any work on it. A procedure might be shoring the truss - maybe in three places and jacking the center some distance above the design bottom chord elevation; installing the new tie rods and anchorage and then removing the shoring. You'll have to calculate the theoritical difference of the deadload deflection with the new configuration.

If there's problems with these trusses, I would shore all of them until the the repairs are completed. One truss failure will probably bring all of them down. (Investigated similar situation in Richmond in the early 90's where one truss failed and because of piping and other junk attached over the years, brought all of the rest of the trusses down. (about 30!)

Interesting item was that there had been a number of repairs over the years of broken webs and split connections (upper and lower) chords that were mickey mouse. Ovisously never reviewed by an engineer or a city inspector. (Trusses installed in the 1950's during the Korean War). During the failure there were six people in the building but because the building stored wreck cars that were being parted out, the trusses came down and rested on top of the cars and nobody was hurt. An argument ensued in the office as to which truss failed first and caused the ensuing ripple effect. I maintained that a certain truss had failed and was challenged how I could state this. After some discussion (understatement), I admitted that I had spoken with an eye witness. Frankly I had no idea which truss failed and there were a number of candidates.

 

[efsinc]

Thank you for the ideas and cautions...fantastic. this is what i have arrived at to date.....

the plan for the bottom chord is to
1) jack and shore the truss at each top chord panel point to the original geometry.
2) remove the heel connection bolts
3) fabricate a new steel plate and rod assembly with plate holes matching the existing heel plate and rod aligned for center of bottom chord
4) install plate/rod assembly each side of each heel with new longer bolts
5) install new full length rod each die of bottom chord attaching heel to heel with rod and turnbuckles
6) complete all other repairs
7) provide tension in rod as designed.
8) remove shores

I am not sure how to verify that the contractor has provided the necessary rod tension. I sure appreciate additional comments.

thank you.

 

[Gillespie]

efsinc,
I had a similar experience with a bowstring truss as 'oldrunner' explains. One truss deflected and the adjacent trusses were overloaded when the continuous rafters added substantial load to the adjacent trusses.
We saved a hell of a lot of our clients money by adding some 2-interior posts and fdn's along the span of the trusses.
Wasn't too disruptive to a grocery store's floor plan, but i don't know what your building use is.
Something too consider, because resolving the forces at the truss heel can be a bitch especially if it is a tight attic space.

 

[Ron]

I realize this thread has been running a while, but can we assume you know the cause of the chord failure?

 

[efsinc]

Ron, (and all),
I am not sure of the immediate cause of the chord failure at this time. Preliminary calcs show all of the bottom chords grossly undersized as is typical for this type of construction at that time according to my understanding. The several broken trusses are at apparently random locations throughout the rather large building. Only two are adjacent to each other such that the load redistribution upon the sagging of the one would have overloaded its adjacent sister (cousin? brother?).

i will be performing a further inspection later this week and reviewing the structure with the insights accumulated from these many posts. Thank you all. I will post more after the inspection.

Thanks!

 

[hokie66]

If you have a wind net uplift load condition, the rods wouldn't help much in compression.

 

[JAE]

Per hokie66's uplift statement - I believe that is why we used steel tubes in our repair (per my earlier post above).

 

[frogit22]

you may want to also look at this thread.

http://www.eng-tips.com/viewthread.cfm?qid=290894